This study is to compare KHCO3 and placebo in attenuating or preventing age-related bone mass decline in postmenopausal women. Percutaneous iliac crest bone biopsies will be done in women who have been taking KHCO3 or placebo for 2 yrs. This is to determine if the new bone being formed duriing KHCO3 administration is histologically normal. This is a cooperative study between our GCRC and the GCRC at UCLA. Bone biopsy specimens will be processed by the CORE lab and sent to UCLA CORE Lab for analysis.